The present invention relates to a process for the preparation of carbonyl chlorides from carboxylic acids, carboxylic anhydrides, cyclic carboxylic esters (lactones) or sulfonic acids.
Carbonyl chlorides are important reactive intermediates in the preparation of fibers, films and in pharmaceutical chemistry and agrochemistry. Thus, a number of preparation processes for the synthesis of carbonyl chlorides from carboxylic acids or carboxylic anhydrides is known. In the industrial preparation, it is possible to differentiate between three methods for the preparation of carbonyl chlorides:
1. the chlorination of carboxylic acids or carboxylic anhydrides with phosphorus trichloride,
2. the chlorination of carboxylic acids or carboxylic anhydrides with thionyl chloride,
3. the chlorination of carboxylic acids or carboxylic anhydrides with phosgene.
The chlorination with phosgene is advantageous as compared with the other processes since only gaseous by-products form, which can be removed easily by expulsion with, for example, nitrogen. In contrast to thionyl chloride, toxic SO2 is not formed, but rather nontoxic CO2. Furthermore, phosgene is a low-cost chlorinating agent. The chlorination with phosgene is therefore a very economical route for the preparation of carbonyl chloride.
Since phosgene on its own is too unreactive at suitable reaction temperatures and pressures, the use of a catalyst is necessary.
The literature describes a number of processes in which N,N-disubstituted formamides or their hydrochlorides are used as phosgenation catalysts. These react with phosgene to give so-called Vilsmeier salts. The Vilsmeier salt, the actual reactive chlorinating reagent, reacts with a carboxylic acid or a carboxylic anhydride to give the acid chloride. In the reaction, formamide hydrochloride is reformed, which in turn can react with phosgene to give the Vilsmeier salt, and passes through further catalyst cycles. The N,N-disubstituted formamide hydrochlorides or their Vilsmeier salts are, however, not entirely thermally stable, meaning that temperatures above from 80 to 90xc2x0 C. can lead to secondary reactions.
EP-A 0 213 976 describes the use of hexaalkylguanidinium salts as catalysts. These need only be added to the reaction mixture in small amounts to achieve adequate selectivity. A disadvantage of this class of compound is, however, its complex preparation.
U.S. Pat. No. 3,547,960 discloses the use of certain catalysts which have Cxe2x80x94N or Nxe2x80x94N double bonds. Inter alia, cyclic amidines are disclosed as catalysts. Preferred catalysts are imidazoles or hydrochloride salts thereof and triazoles.
It is an object of the present invention to provide catalysts for the reaction of carboxylic acids, carboxylic anhydrides, cyclic carboxylic esters (lactones) or sulfonic acids with phosgene to give carbonyl chlorides which can be prepared more readily than known catalysts and can be used for a large number of compounds. Furthermore, the catalysts should permit shorter reaction times than the catalysts known hitherto and very good conversions.
We have found that this object is achieved by a process for the preparation of acid chlorides by reaction of carboxylic acids, carboxylic anhydrides, cyclic carboxylic esters(lactones) or sulfonic acids with phosgene in the presence of a catalytic amount of a compound from the group consisting of N,N,Nxe2x80x2,Nxe2x80x2-tetrasubstituted amidinium halides (I), N,N,Nxe2x80x2-trisubstituted amidinium hydrohalides (II) and N,N,Nxe2x80x2-trisubstituted amidines (III) of the formula 
in which R1, R2 and R4 are linear or branched alkyl chains having a length of from 1 to 20 carbon atoms or cycloaliphatic radicals having a ring size of from 5 to 8 carbon atoms, where the rings can be interrupted by heteroatoms, or R1 and R2 are unsubstituted or substituted aromatic radicals or together form a chain of four or five methylene groups;
R3 is a hydrogen atom or a branched or unbranched alkyl radical or a cycloalkyl radical having a length of from 1 to 6 carbon atoms; and
R5 in compound I is a branched or unbranched C1 to C6-alkyl chain, and Xxe2x88x92 is a halide.
Thus, R1, R2 and R4, and also R5, can, for example, independently of one another be alkyl radicals such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl or linear or branched pentyl radicals. The radicals R1, R2 and R4 can also be cycloalkyl radicals, such as cyclopentyl and cyclohexyl radicals, and R1 and R2 can be aromatic radicals, substituted or unsubstituted, such as phenyl and tolyl radicals.
R3 can, for example, be the same alkyl radicals as listed for R1, R2, R4 and R5. Preferably, however, R3 is hydrogen. Xxe2x88x92 is preferably bromide or chloride, very particularly preferably chloride.
Particularly preferably, the radicals R1, R2, R4 and R5 are methyl or ethyl radicals, and R3 is hydrogen. Particular preference is given to N,N,Nxe2x80x2-trimethy-formamidine (R1=R2=R4=methyl, R3=H), N,N,Nxe2x80x2-trimethylformamidine hydrochloride (R1=R2=R4=methyl, R3=H),N,N,Nxe2x80x2,Nxe2x80x2tetramethylformamidinium chloride (R1=R2=R4=R5=methyl, R3=H) and N,N-diethyl-Nxe2x80x2,Nxe2x80x2-dimethylformamidinium hydrochloride (R1=R2=methyl, R4=R5=ethyl, R3=H).
The catalysts used in the process according to the invention are also obtainable on a larger scale by simple reaction. They permit reaction in very good yields and short reaction times.
The N,N,Nxe2x80x2,Nxe2x80x2-tetrasubstituted formamidinium salts (I) (R3=H) are obtainable via a single-stage reaction from formamides with N,N-disubstituted dialkylcarbamoyl halides. A synthesis of the compounds is described in Coll. Czech. Chem. Comm. 24 (1959), 760 to 765. N,N,Nxe2x80x2-Trisubstituted amidinium hydrohalides (II) can be prepared by reaction of N-monosubstituted amides with N,N-dialkylcarbamoyl halides in a single-stage synthesis in accordance with Kantlehner et al., Synthesis 1983. The free amidines (III) can be liberated by neutralization of the resulting amidine hydrohalides using an inorganic base.
Since the preparation of the free amidines (III) requires an additional reaction step, namely the neutralization of the hydrohalides (II), in the process according to the invention preference is given to using the amidinium hydrohalides of the formula II.
In a further preferred embodiment, N,N,Nxe2x80x2,Nxe2x80x2-tetrasubstituted amidinium halides (I), particularly preferably formamidinium halides, are used, which are likewise obtainable via a single-stage reaction, as already described.
The catalytic action of the amidinium salts I and II in the process according to the invention is surprising since they have hitherto been considered to be reaction-inhibiting. For example, U.S. Pat. No. 3,547,960 states that a disadvantage of processes in which carboxamides are used as catalysts is that tarry, catalytically inactive products are formed during this reaction as a result of the decomposition of the carboxamide catalysts. From the literature, however, it is known that formamidinium halides form as decomposition product of the carboxamide DMF (dimethylformamide).
The reactions in which amidines, amidinium hydrohalides and amidinium halides are used are more successful the more substituents the two nitrogen atoms have. The reaction does not proceed to completion if substituents are not present (Comparative Examples 5 and 6). With two substituents on the nitrogen atoms, the reaction proceeds slowly and relatively large amounts of the carboxylic anhydride are formed (Comparative Example 7).
The carboxylic acids which can be used in the process according to the invention are not restricted. In general, use is made of aliphatic carboxylic acids having from 2 to 22 carbon atoms or mixtures of C8-C22 carboxylic acids, the radicals of which can be branched or linear, saturated or unsaturated and optionally substituted by, for example, halogen or nitro groups. Furthermore, it is possible to use aromatic and cycloaliphatic carboxylic acids, and aralkyl- or alkylaryl-substituted carboxylic acids having from 7 to 24 carbon atoms. Suitable acids can comprise from one to three carboxyl groups. Suitable aliphatic acids are, for example, pivalic acid, 2-ethylhexanoic acid, stearic acid, butyric acid, lauric acid, palmitic acid, acetic acid, neopentanoic acid, chloroacetic acid, dichloroacetic acid, adipic acid, sebacic acid, acrylic acid, methacrylic acid etc. Suitable aromatic acids are, for example, benzoic acid, m-nitrobenzoic acid, isophthalic acid, phenylacetic acid, p-chlorobenzoic acid, trans-cinnamic acid, m-toluic acid etc. An example of a suitable cycloaliphatic acid is cyclohexanecarboxylic acid.
Suitable carboxylic anhydrides for the process according to the invention are generally anhydrides of the formula (IV) 
in which Rxe2x80x2 and Rxe2x80x3 are each an organic radical, for example a hydrocarbon radical. Particularly suitable are carboxylic anhydrides which carry aliphatic groups, which can be branched or linear, saturated or unsaturated and optionally substituted by halogen or nitro groups, or cycloaliphatic or aromatic groups, and aralkyl or alkylaryl groups. Thus, Rxe2x80x2 or Rxe2x80x3 can be alkylene, alkenylene, cycloalkylene, arylene or a divalent, saturated or unsaturated group. The number of carbon atoms in the aliphatic radicals Rxe2x80x2 and Rxe2x80x3 is preferably from 1 to 24, particularly preferably from 1 to 12, and in aromatic, cycloaliphatic, aralkyl or alkylaryl radicals is from 6 to 24, preferably from 6 to 12. Thus, anhydrides may, for example, be acetic anhydride, butyric anhydride, hexanoic anhydride, benzoic anhydride, trimellitic anhydride, octanoic anhydride, chloroacetic anhydride, acrylic anhydride, phenylacetic anhydride, adipic anhydride, sebacic anhydride, nitrobenzoic anhydride, chlorobenzoic anhydride, toluic anhydride, isophthalic anhydride and terephthalic anhydride.
Suitable cyclic carboxylic esters (lactones) for the process according to the invention are generally cyclic esters of the formula V: 
in which n=2 to 9.
In addition, mono- or polysubstituted cyclic carboxylic esters in which one or both hydrogen atoms of one or more CH2 groups have been replaced by substituents, in particular alkyl groups, are also suitable.
Particular preference is given to unsubstituted cyclic carboxylic esters, very particularly preferably where n=3 to 5. Thus, for example, xcex3-butyrolactone, xcex4-valerolactone and xcex5-caprolactone are suitable.
The catalyst is generally used in the process according to the invention in an amount of from 0.1 to 5 mol %, preferably from 0.5 to 3 mol %, particularly preferably from 1 to 2 mol %, based on the amount of carboxylic acid used, of the carboxylic anhydride used, of the cyclic carboxylic ester used or of the sulfonic acid used.
The process according to the invention can be carried out in the absence or in the presence of an organic solvent which is inert toward phosgene. Preferred solvents are hydrocarbons. Particular preference is given to mono- or polysubstituted aromatic hydrocarbons, very particular preference to toluene, o-, m-, p-xylene or chlorobenzene.
In a particularly preferred variant, in particular during reaction of cyclic carboxylic esters (lactones), the desired carbonyl chloride is used as solvent.
However, the absence of additional solvent is preferred over carrying out the process according to the invention in the presence of a solvent. This avoids separating the solvent off from the reaction product and thus an additional process step.
The process can be carried out in any apparatus suitable for the reaction. Thus, for example, it is possible to use a phosgenation apparatus with attached carbonic acid condenser. In this connection, the process can be carried out discontinuously or continuously, preferably continuously. The reaction temperature in the process according to the invention is dependent on the carboxylic acid used or on the carboxylic anhydride used. Generally, the process according to the invention is carried out at temperatures of from 40 to 150xc2x0 C., preferably from 60 to 120xc2x0 C., particularly preferably from 80 to 100xc2x0 C. The reaction pressure is unimportant for the reaction. For the most part, the reaction is carried out at a pressure of generally from 0.5 to 5 bar, preferably from 0.5 to 2 bar, particularly preferably from 0.8 to 1.2 bar, very particularly preferably at atmospheric pressure.
The reaction times are in the range from 1 to 8 hours, preferably in the range from 2 to 4 hours, depending on the amount and type of starting compound used. The GC yields are generally greater than 90%, preferably greater than 95%, particularly preferably greater than 98%. Here, GC yields are yields determined by gas chromatography. The conversions are usually virtually complete.
The phosgene can be added to the reaction mixture in gaseous form or in condensed form.
In a preferred embodiment, the carboxylic acid or the carboxylic anhydride and the catalyst are initially introduced and heated to the reaction temperature. Phosgene is then gassed in or condensed in until the reaction is complete or until the amount of phosgene gassed in or condensed in corresponds at most to twice the molar amount of carboxylic acid used or of carboxylic anhydride used. In general, for a complete conversion, a total of from 1.0 to 2.0 equivalents, preferably from 1.1 to 1.5 equivalents, based on the molar amount of carboxylic acid used or carboxylic anhydride used, of phosgene are required. The acid chlorides formed can be isolated and worked up using methods known to the person skilled in the art, such as distillation, precipitation and filtration and recrystallization etc. The acid chlorides are preferably separated off by simple decantation from the catalyst, which is insoluble in the acid chloride. Such a work-up is simple and low-cost and the catalyst employed can be reused.
The examples below further illustrate the invention.